Mindaugas Pranevicius

pH-dependent modulation of voltage gating in connexin45 homotypic and connexin45/connexin43 heterotypic gap junctions

Intracellular pH (pHi) can change during physiological and pathological conditions causing significant changes of electrical and metabolic cell–cell communication through gap junction (GJ) channels. In HeLa cells expressing wild-type connexin45 (Cx45) as well as Cx45 and Cx43 tagged with EGFP, we examined how pHi affects junctional conductance (gj) and gj dependence on transjunctional voltage (Vj). To characterize Vj gating, we fit the gj–Vj relation using a stochastic four-state model containing one Vj-sensitive gate in each apposed hemichannel (aHC); aHC open probability was a Boltzmann function of the fraction of Vj across it. Using the model, we estimated gating parameters characterizing sensitivity to Vj and number of functional channels. In homotypic Cx45 and heterotypic Cx45/Cx43-EGFP GJs, pHi changes from 7.2 to ~8.0 shifted gj–Vj dependence of Cx45 aHCs along the Vj axis resulting in increased probability of GJ channels being in the fully open state without change in the slope of gj dependence on Vj. In contrast, acidification shifted gj–Vj dependence in the opposite direction, reducing open probability; acidification also reduced the number of functional channels. Correlation between the number of channels in Cx45-EGFP GJs and maximal gj achieved under alkaline conditions showed that only ~4% of channels were functional. The acid dissociation constant (pKa) of gj–pHi dependence of Cx45/Cx45 GJs was ~7. The pKa of heterotypic Cx45/Cx43-EGFP GJs was lower, ~6.7, between the pKas of Cx45 and Cx43-EGFP (~6.5) homotypic GJs. In summary, pHi significantly modulates junctional conductance of Cx45 by affecting both Vj gating and number of functional channels.

Transition to Collateral Flow After Arterial Occlusion Predisposes to Cerebral Venous Steal

Background and Purpose— Stroke-related tissue pressure increase in the core and penumbra determines regional cerebral perfusion pressure (rCPP) defined as a difference between local inflow pressure and venous or tissue pressure, whichever is higher. We previously showed that venous pressure reduction below the pressure in the core causes blood flow diversion–cerebral venous steal. Now we investigated how transition to collateral circulation after complete arterial occlusion affects rCPP distribution. Methods— We modified parallel Starling resistor model to simulate transition to collateral inflow after complete main stem occlusion. We decreased venous pressure from the arterial pressure to zero and investigated how arterial and venous pressure elevation augments rCPP. Results— When core pressure exceeded venous, rCPP=inflow pressure in the core. Venous pressure decrease from arterial pressure to pressure in the core caused smaller inflow pressure to drop augmenting rCPP. Further drop of venous pressure decreased rCPP in the core but augmented rCPP in penumbra. After transition to collateral circulation, lowering venous pressure below pressure in the penumbra further decreased rCPP and collaterals themselves became a pathway for steal. Venous pressure level at which rCPP in the core becomes zero we termed the “point of no reflow.” Transition from direct to collateral circulation resulted in decreased inflow pressure, decreased rCPP, and a shift of point of no reflow to higher venous loading values. Arterial pressure augmentation increased rCPP, but only after venous pressure exceeded point of no reflow. Conclusions— In the presence of tissue pressure gradients, transition to collateral flow predisposes to venous steal (collateral failure), which may be reversed by venous pressure augmentation.

Application of Stochastic Automata Networks for Creation of Continuous Time Markov Chain Models of Voltage Gating of Gap Junction Channels

The primary goal of this work was to study advantages of numerical methods used for the creation of continuous time Markov chain models (CTMC) of voltage gating of gap junction (GJ) channels composed of connexin protein. This task was accomplished by describing gating of GJs using the formalism of the stochastic automata networks (SANs), which allowed for very efficient building and storing of infinitesimal generator of the CTMC that allowed to produce matrices of the models containing a distinct block structure. All of that allowed us to develop efficient numerical methods for a steady-state solution of CTMC models. This allowed us to accelerate CPU time, which is necessary to solve CTMC models, ~20 times.

Continuous Time Markov Chain Models of Voltage Gating of Gap Junction Channels

The major goal of this study was to create a continuous time Markov chain (CTMC) models of voltage gating of gap junction (GJ) channels formed of connexin protein. This goal was achieved by using the Piece Linear Aggregate (PLA) formalism to describe the function of GJs and transforming PLA into Markov process. Infinitesimal generator of CTMC was used to automate construction of Markov chain model from description of the system using PLA formalism. Developed Markov chain models were used to simulate gap junctional conductance dependence on transjunctional voltage. The proposed method was implemented to create models of voltage gating of GJ channels containing 4 and 12 gates. CTMC modeling results were compared with the results obtained using a discrete time Markov chain (DTMC) model. It was shown that CTMC modeling requires less CPU time than an analogous DTMC model. DOI: http://dx.doi.org/10.5755/j01.itc.43.2.3198

Severe Intraoperative Bronchospasm Treated with a Vibrating-Mesh Nebulizer

Bronchospasm and status asthmaticus are two of the most dreaded complications that a pediatric anesthesiologist may face. With the occurrence of severe bronchospasm and the inability to ventilate, children are particularly vulnerable to apnea and ensuing hypoxia because of their smaller airway size, smaller lung functional residual capacity, and higher oxygen consumption rates than adults. Nebulized medication delivery in intubated children is also more difficult because of smaller endotracheal tube internal diameters. This case demonstrates the potentially lifesaving use of a vibrating-mesh membrane nebulizer connected to the anesthesia circuit for treating bronchospasm.

Comment on: High stop-bang questionnaire scores predict intraoperative and early postoperative adverse events.

Dear Sir, With great interest, we read the article by Seet et al,(1) which delved into the ongoing controversy of how the risk of perioperative respiratory events is related to specific STOP-BANG scores. This controversy is fuelled by the belief that higher STOP-BANG scores translate to higher risks of OSA and hence, an increased incidence of postoperative complications.(2-4) Seet et al’s study also supports this belief; using logistic regression analysis, the authors analysed the cohort for the odds of adverse postoperative events occurring and found a statistically significant increase in the odds of an adverse event in patients with STOP-BANG scores of 2, 3, 4, 5 and ≥ 6. Moreover, they concluded that a STOP-BANG score ≥ 5 resulted in a five-fold increased odds ratio (OR) of adverse events and recommended that polysomnography be considered for these patients before elective surgery. However, upon reviewing the confidence intervals for the reported ORs, we noticed a significant overlap for most STOP-BANG score groups. Therefore, we re-analysed the study data with analysis of variance as well as performed a pairwise comparison between the groups using the Bonferroni correction. Our analysis showed that only two groups (that are separated by a STOP-BANG score of 3) have different risks for postoperative complications; adding points to the STOP-BANG score did not statistically alter the risk (Table I). Therefore, we suggest that a STOP-BANG score ≥ 3 should trigger an evaluation for sleep-disordered breathing. Table I Association of adverse events with STOP-BANG score (adapted from Seet et al).(1) Yours sincerely,

Clinical Assessment of Noninvasive Intracranial Pressure Absolute Value Measurement MethodAuthor ResponseAuthor Response

Ragauskas et al.1 demonstrated that when external pressure is applied, the orbit tends to equilibrate flow in the ipsilateral ophthalmic artery (OA) when its level approaches intracranial pressure (ICP). The authors detected this relationship by making the OA into a “natural pair of scales, in which the intracranial segment of the OA is compressed by extracranial pressure (Pe) applied to the orbit.”